Photosensitive elements are used in a wide variety of applications in the graphic arts and electronics industries. Such elements are used, for example, to prepare printing plates, proofing elements, photoresists, and the like. Photosensitive elements generally comprise a support and a photosensitive layer provided thereon. The element is typically exposed to actinic radiation through a transparency or phototool to produce exposed areas and nonexposed areas. Exposure to actinic radiation results in a change in a physical property of the exposed areas of the photosensitive layer, e.g., a change in solubility, swellability or dispersibility, a change in tackiness, a change in adhesion, a change in color, etc., so that the exposed and nonexposed areas can be differentiated in a subsequent development step. Development may involve (a) removal of the nonexposed areas which are more soluble, swellable or dispersible by applying a suitable solvent to the element, (b) application of a colored toning material which adheres to the more tacky areas, or (c) removal of less adherent material by transfer or peel-apart, etc.
The support in the photosensitive element is generally a thin metal sheet or, more commonly, a flexible, transparent polymeric film, such as polyester. In some applications, the support must remain firmly bonded to the photosensitive material throughout the life of the product, including the development step. In other applications, the support must be removable. In many cases, the desired degree of adhesion is achieved by first coating the film support with an adhesion-adjusting material, i.e., a material which either increases or decreases the adhesion.
There are problems associated with the use of adhesion-adjusting materials. For example, frequently it is difficult to obtain an adhesive material which bonds firmly to both the support and to the overlying photosensitive layer. In some cases, the adhesive is subject to solvent attack so that the initially high adhesion to the support is significantly reduced when the photosensitive layer is applied as a solvent coating. Similarly, there are problems with existing materials which decrease adhesion. For example, it is difficult to obtain a material which bonds firmly to the support, but releasably to the overlying photosensitive layer. Silicone release layers are commonly used with polyester supports, however, on occasion, some silicone material remains on the back side of the overlying layer when it is peeled off the polyester support.
European Patent application 416,765 A2 published Mar. 13, 1991, describes a release film comprising a polymeric film substrate and a polymeric abherent layer. The abherent layer comprises a polyurethane resin which is the reaction product of (i) an organic polyisocyanate (ii) an isocyanate-reactive polydialkyl siloxane and (iii) a polymeric polyol. The abherent layer adheres firmly to the film substrate but provides release from an opposing surface.
It is desirable to obtain a film support that has adjustable adhesion, i.e., a film support which can be easily removed from the photosensitive layer or remain firmly adhered to the photosensitive layer, depending on the final use. A film support having adjustable adhesion is particularly desirable for photosensitive elements which are used to make relief images for printing plates. A need also exists for a film support that overcomes the disadvantages associates with the use of conventional adhesion-adjusting materials.
These needs are met using the photosensitive element of the invention.